Thermostatic switch



Jan. 31, 1961 s, P R, JR 2,970,195

THERMOSTATIC SWITCH Filed Dec. 8, 1958 FRE D S. KEEF 511,

2,970,195 THERMOSTATIC SWITCH Fred S. Keefer, Jr., Chatham, N.J., assignor to The Chatham Controls Corporation, Chatham, N.J., a corporation of New Jersey Filed Dec. 8, 1958, Ser. No. 778,929 '1 Claim. (Cl. 200-138) The invention relates in general to thermostatic switches employed to control the operation of various types of apparatus by opening and closing an appropriate electric circuit in each instance in response to changes in ambient temperature, and has particular reference to a switch of the class disclosed in prior United States Patent No. 2,773,962, issued December 11, 1956, to Dudley C. Perst, wherein the cooperative electrical contacts are caused to rub together evenly while in their normally closed condition either before opening or after closing to insure clean, electrically perfect contact faces throughout the expected maximum life of such a switch.

In accordance with the disclosure in said prior patent, there is an elongated tubular casing made of a metal having a high coefficient of thermal expansion, such as brass. Inside this casing, there is a similarly elongated, axially extending bimetallic thermostatic element that has one end free for radial movement under the influence of temperature changes and has its opposite base end rigidly supported in a closure cap for one end of the casing, whose other end is closed by an integral end wall. The base end of the bimetallic element is electrically insulated from the closure cap in suitable manner, such as by being embedded in a block of glass insulation material, and has a stem-like electrical terminal member projecting through the casing end cap to the exterior for use as one circuit-connectable terminal of the switch. A movable electrical contact is movable radially with the free end of the bimetallic element and is maintained in radial registration with a stationary electrical contact supported by the casing and being in electrical connection with the latter, whereby the casing is included in the electric circuit, which in the beginning was considered to be desirable in the interest of switch miniaturization.

It has been found, subsequent to issuance of said prior patent, that, on the contrary, it is undesirable to include the casing in the same circuit with the electrical contacts, because, whenever the thermostatic switch is installed in a metallic frame or other device, the switch casing will become grounded electrically in said frame. In various other installations, too, the switch casing will be exposed for possible short-circuiting contact with other circuits when bridged by hand-held tools or the like. This condition has proved to be highly objectionable and even dangerous.

In view of the above-enumerated disadvantages of the circuit-included condition of the casing of said patented thermostatic switch, it is the primary object of the present invention to provide an improved structure wherein a new combination of elements exists. To be more explicit, the stationary contact element of the combination is supported by a regulating screw for radial adjustment of said stationary contact in relation to the movable contact and the regulating screw is in screwthreaded engagement with a metallic insert which penetrates the side wall of the casing but is electrically insulated therefrom. An electrical terminal member, which cooperates with the first-mentioned terminal member that protrudes exteriorly from the bimetallic element at the capped end of the casing, is connected to the metallic insert on the outside of the casing. Due to this new arrangement, the casing is insulated from the electric circuit and will not nitecl States Patent be energized when the circuit is closed. Moreover, the outer end of the regulating screw that supports the interiorly located stationary contact is exposed for convenience in application of a screwdriver or other adjusting tool.

Further objects, advantages and features of the invention will become apparent as the following specific description is read in connection with the accompanying drawing, in which:

Fig. l is a large-scale longitudinal section of the device in closed-circuit condition; Fig. 2 is an end elevation; and Fig. 3 is a top plan View.

Referring now in detail to the drawing, wherein like reference characters designate corresponding parts in the several views, Figs. 1 to 2 represent an exemplary embodiment of the invention that is direct-acting in principle of operation, i.e. the electrical contacts are normally in closed condition and will be caused to open a control circuit only upon the occurrence of a rise in ambient temperature.

The device includes a substantially thimble-shaped, elongated, cylindrical casing 10 having one end closed by an integral end wall 11. Casing 10 is made of a metal possessing a comparatively high coeificient of thermal expansion, such as brass, in order that it may serve a useful purpose in connection with contact-rubbing action to be described in detail later herein.

As in said earlier device, the originally open end of casing 10 has a metallic cap 112 fitted therein with a pressed joint and united in hermetically sealed relation thereto by suitable means, such as solder. A stem-like first electrical terminal member 13 is mounted in centered penetrating relation to cap 312 of casing 10 in a molded block 14 of electrical insulation material and protrudes to a suitable distance from the capped end of the casing for convenient connection to one side of a control circuit in which the switch may be installed. By using glass as the insulation material, a glass-to-metal base is formed for casing 10 to provide a hermeticseal as well as insulation.

A bimetallic thermostatic element 15, which preferably is in conventional strip form and in substantially straight condition at room temperature, has its base end afiixed to the inner end of terminal member 13 in electrical connection therewith, as by soldering, brazing or welding processes. Bimetallic element 15 extends along the axis of casing 10 in spaced relation to its side wall 16 and is composed of two integrally united components 15' and 15" possessing widely separated coefiicients of thermal expansion in accordance with conventional practice, so that the'said element will bend laterally, or radially with respect to the casing axis, when subjected to a change in ambient temperature. Consequently, by making component 15' of invar, which has a low coefiicient of expsansion, and compoent 15" of iron, whose coefiicient of expansion is high as compared to that of invar, element 15 as a whole will bend downward in the arrangement shown in Fig. 1 when the ambient temperature rises.

There are two coacting electrical contacts 17 and IS in the switch. These contacts preferably are both made of silver or platinum because of the well known high conbimetallic element 15. It may be aflixed directly tobimetallic element 15 as shown in Fig. 1 or in an equivalent manner may be supported by a spring tongue (not shown) that bears against bimetallic element 15 and is moved 3 thereby as shown in Fig. 4 of the drawing in said p rior patent. The other contact 18 will be called the stationary contact because it is supported in stationary position by casing wall 16 through the medium of means which will now be described.

Stationary contact 18 is aflixed by brazing, or in some other suitable manner to the inner end of a radial regulating screw 19 in electrical connection therewith. Screw 19 penetrates, and has screwthreaded engagement with, a cylindrical metal insert 20. This insert, in turn, projects radially through a metallic sleeve, or bushing, 21 that is countersunk in an opening 22 in side wall 16 of casing and is electrically insulated from the marginal edge of said opening by a concentric intervening ring 23 of electrical insulation material, such as glass. The areas of joinder between the respective concentric parts, viz: screw 19, insert 20, sleeve 21, insulation ring 23, and the marginal edge of casing opening 22, should be sealed hermetically in some suitable manner in order that the interior of casing 10 may be rendered proof against ingress of gas, dust and moisture. The outer end of regulating screw 19 is provided with a transverse kerf 24 for application of a screwdriver whenever it is necessary to adjust the position of stationary contact 18 in relation to movable contact 17. A second terminal member 25 in the form of an arm radial to the axis of screw 19 has an annular hub 26 affixed in suitable manner to the outer end of insert 20. Terminal member 25 thus is located outside casing 10 for cooperation with terminal member 13 as means by which the thermostatic switch may be connected to the opposite sides or poles of an electric control circuit.

Obviously, when the thermostatic switch is to be directacting, as shown, the iron component of bimetallic element 15, which has the higher coei'licient of thermal expansion, will be arranged to face stationary contact 18 and the other component 15', having the lower coefiicient of expansion, will face away from contact 18. As a result of this arrangement, when the ambient temperature rises, the free end of bimetallic element 15 will curl away from stationary contact 18 and move movable contact 17 away from contact with said contact 18 to open the control circuit. However, it is very easy to convert the thermostatic switch into an inverse-acting switch, which may be accomplished simply by rotating the cap 12 through 180 degrees prior to assembly with casing 10, in order to reverse the components 15 and 15". Prior to assembly, bimetallic element 15 as a whole should be bent laterally to such a degree that contacts 17 and 18 will be normally separated at room temperature into open-circuit condition. This contact adjustment may be found to be only roughly effective but may be corrected by precise adjustment of regulating screw 19.

The contact rubbing and cleaning action which is peculiar to the general structure of both said earlier thermostatic switch and the one disclosed herein will now be described.

Casing 10 is made of brass or some other metal having a high coefiicient of thermal expansion and is so elongated that its length will increase to an appreciable degree when subjected to a rise in ambient temperature. During the time interval required for the rise in temperature to be conducted through casing 10 and the air content of its interior chamber to bimetallic element 15, stationary contact 18 will be moved lengthwise of casing 10 in rubbing relation to movable contact 17 regardless of the metallic composition of said element 15. However, because the dissimilar metals of which bimetallic element 15 is composed both have coefficients of expansion lower than that of brass casing 10, there will be appreciable relative axial movement in said casing between contacts 17 and 18 and resulting rubbing and cleaning action even under conditions of unexpectedly severe rises in ambient temperature in which heat transmission from casing 10 to bimetallic element 15 will be greatly accelerated. In

any event, the tip of bimetallic element 15 cannot be curled laterally to open contacts 17 and 18 until after some axial rubbing has had time to occur.

In a thermostatic switch wherein the arrangement of bimetallic element components is intended for inverse action, contraction in length of the casing following closure of the contacts will cause rubbing and cleaning of the latter.

While the contact cleaning action in my improved thermostatic switch is the same as in said prior patented device, the point of novelty in the present invention is the new combination of structural elements whereby casing 10 is insulated from both of the terminal members 13 and 25 with elimination of the inconveniences and hazards inherent in the earlier device wherein the stationary con tact was electrically connected to the casing.

It will be understood that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit of the invention and scope of the appended claim.

I claim:

A thermostatic switch comprising: an elongated, substantially straight, terminally closed, air-containing, tubular casing composed of a metal possessing a high coetficient of thermal expansion; respective first and second electrical terminal members penetrating one end wall and the side wall of said casing respectively and having exteriorly projecting portions for connection to an electrical control circuit; means to insulate said first terminal member from said casing; an elongated, substantially straight, bimetallic thermostatic element having a free end and a base end electrically connected to said first terminal member and being supported thereby in axially extending position in such a manner as to be insulated from the casing, said bimetallic element being constructed and arranged in such a manner that the free end thereof will move radially in the casing under the bending influence of changes in ambient temperature; a movable electrical contact movable radially with the free end of the bimetallic element; a stationary contact connected electrically to said second terminal member in insulated relation to the casing and being in radial registration with the movable contact; the side wall of the casing having a radial opening substantially at the location of said second terminal member; a metallic bushing countersunk in said radial opening; a cylindrical metallic insert projecting radially from the interior of the casing through the center of said metallic bushing and being spaced from the latter and having said second terminal member afiixed to the outer end thereof in electrical connection therewith; an insulating ring intervening between said metallic bushing and metallic insert in coaxial relation thereto; and a screw of electrical conducting material having screwthreaded engagement with said metallic insert in penetrating relation thereto and being arranged with its inner end in radial registration with the movable contact and its outer end exposed for engagement by a turning tool; the stationary contact being supported by the inner end of said screw; the construction and arrangement of the casing, the bimetallic element and the electrical contacts being such that conduction of heat between said casing and bimetallic element will be retarded sufiiciently during each time period when the contacts are in closed condition that elongation of the casing through the influence of ambient temperature changes will cause axial rubbing of said contacts and consequent cleaning action.

References Cited in the file of this patent UNITED STATES PATENTS 2,146,604 Taylor Feb. 7, 1 939 2,328,342 Howard Aug. 31, 1943 2,767,284 Moksu Oct. 16, 1956 2,773,962 Perst Dec. 11, 1956 2,906,840 Ulanet Sept. 29, 1959 

